Endodontic therapy is a debridement procedure that requires removal of the irritants from the canal and periapical tissues if the treatment is to be successful. It is well established that bacteria are the main etiological factors in the development of dentinal caries and its progression to pulpal and periapical disease. E. faecalis is the bacterial species most frequently recovered from root-filled teeth. Studies have shown that E. faecalis is able to withstand a high alkaline environment such as the one generated by calcium hydroxide. The resistance appears to be related to a cell proton-pump that is necessary for survival of the bacterium at high pH. Therefore, E. faecalis is able to form biofilms even in calcium-hydroxide-medicated root canals. In addition, under starved conditions, this resilient organism shows tolerance to sodium hypochlorite, heat, hydrogen peroxide, acid and ethanol. E. faecalis can also survive extended periods of starvation in water, within water-filled dentinal tubules, and in human serum.